Practical Microbiology: Strep Typing

         Beta-hemolytic streptococci (BHS) can cause a lot of issues in patients. At work, we have dedicated cultures for screening Group A (strep throat) and Group B Strep (labor). Of course, they can cause infections in other parts of the body.

         Unlike many other tests carried out by the lab, Strep. typing is one of the least time consuming tasks. For identification of BHS, we do not rely on the Bruker MALDI-TOF method. The said method cannot identify BHS with high confidence. It's not uncommon for the MALDI to call Group A Strep when it is, in fact, not. Furthermore, that method cannot differentiate between some of them like Group C and G.

         From my experience, the MALDI is good for Groups B, H, and K. Can the technology improve in the future? I say yes.

         For a more definitive identification, where I work opts for latex-agglutination method. We use the Remel™ PathoDX™ Strep Grouping Kits. As the name of the method suggests, it uses IgG antibodies specific to each BHS's antigens. (See the package insert for more details).

         In short, you take the bacteria from the culture and extract their antigens with Reagents 1, 2, and 3. Then, you add the latex to see if there is any agglutination. A reaction means you have found the Lancefield grouping of the bacteria.

         The package insert makes it seem as if you would have to test them with all the latex per organism. The truth is, a microbiologist has an idea what the bacteria might be by their morphology on the plates. Thus, reducing waste and time. For cultures looking for a specific BHS, you would only use one type of latex anyway.

         While the kit can identify BHS off of broth (liquid) media, we do not use it in that capacity. The protocol calls for subbing the bacteria onto solid media. Why? Because it is impossible to tell if you have a pure culture in liquid broth with naked eyes. Solid media would allow you to streak for isolation in case of multiple organisms.

         How long does this take? Not very long. You could do half a dozen patient cultures in under 10 minutes. Remel's kits allow for the identification of Group A, B, C, F, and G.

What's Next?

         For noninvasive infections, the work on the bacteria is complete. In those cases, we attach a note for the clinicians to use beta-lactam antibiotics.

         For invasive infections, those of sterile sites in your body, they need additional work up. This is where the lab would perform antibiotic sensitivity testings. This is usually carried out using the BD Phoenix™'s MIC panels.

         The local and state departments also want notifications of invasive incidents. In the State of Utah, Groups A and B fall into that category. For more information, visit http://health.utah.gov/epi/reporting/Rpt_Disease_List.pdf.

         They track the stats to determine if an outbreak is happening. I suppose that's also how they decide if certain PSAs would need to get out for preventive measures.